1. Field of the Invention
This invention generally relates to etching copper and copper-containing alloys and, more especially, to a new and improved ammoniacal alkaline cupric etchant solution for, and method of, decreasing the amount of etchant undercut on substrates in which copper or copper-containing alloys are present.
2. Description of the Prior Art
Traditionally, acid based etchants were generally employed during the emerging years of the copper printed circuit industry. This was a natural evolution from the experience gained with etchants designed originally for photo-engraving applications. However, the requirements of copper printed circuit makers were different from those of photo-engravers, and acid etchant formulations were modified to fill their needs. As a result, during those early years, ferric chloride, chrome/sulfuric acids and ammonium persulfate were predominant as electronic grade etchants for the printed circuit industry.
During the mid-sixties, a new approach to dissolving metallic copper appeared. This new technology was not based on the traditional acid systems but on an alkaline system. These new alkaline etchants were aqueous ammoniacal solutions containing carbonate ions and a potent oxidizing agent, sodium chlorite. This early alkaline etchant successfully etched copper circuit boards without significantly attacking solder plate, but exhibited a limit etch rate and a limited total copper loading capacity.
In the early nineteen seventies alkaline etchants were further improved in etch rate and total copper loading capacity by the addition of chloride ions to essentially the same ammoniacal, chlorite-containing etchants previously described.
The three-step oxidation mechanism of metallic copper resulting with these alkaline systems is best understood by studying the following three equations. EQU 2Cu.degree.+NaClO.sub.2 +4NH.sub.4 Cl+4NH.sub.4 OH.fwdarw.2Cu(NH.sub.3).sub.4 Cl.sub.2 +NaCl+6H.sub.2 O A.
Equation A illustrates the formation of cupric ions from the reaction of chlorite on metallic copper. EQU Cu.degree.+Cu(NH.sub.3).sub.4 Cl.sub.2 .fwdarw.2Cu(NH.sub.3).sub.2 Cl B.
Equation B illustrates the use of the just-formed cupric ion from Equation A as an oxidizing agent on additional metallic copper to form cuprous ions. EQU 2Cu(NH.sub.3).sub.2 Cl+1/2O.sub.2 +2NH.sub.4 Cl+2NH.sub.4 OH.fwdarw.2Cu(NH.sub.3).sub.4 Cl.sub.2 +3H.sub.2 O C.
Equation C illustrates the secondary oxidation of the cuprous ions formed in Equation B via aeration to useful cupric ions for further metallic copper etching.
From the foregoing chemical reactions, one can see the basic elements contributing to the modern steady state ammoniacal alkaline cupric etching systems. If one has an active etchant, based on cupric ions, and a suitable cupric ion level can be maintained by aerial oxidation of cuprous ions formed in situ during the etching of metallic copper, the potential of replenishable alkaline etchant becomes clear. As metallic copper is etched, cuprous ions are formed and aerially oxidized to the active etchant species, i.e. cupric ions. While this is occurring fresh non-copper-containing replenisher is added. This replenisher is so formulated as to refortify those etchant components that are depleted during etching. The replenisher addition rate is based on a change of an etchant characteristic, e.g. density, as a function of increasing copper concentration. Representative uses of ammoniacal alkaline cupric etching solutions may be found in U.S. Pat. Nos. 3,705,061; 3,717,521; 3,753,818; 3,809,588 and 3,919,100.
The alkaline ammoniacal cupric etching solutions previously discussed are the most frequent type of solution used for etching microcircuits and printed circuit boards due to the fact that they can maintain a high sustained etching rate by the addition of the aforementioned simple non-copper-containing replenisher solution.
Ammoniacal alkaline cupric etchant solutions dissolve copper not only in a generally vertical direction (perpendicular to the planar substrate), as is desired, but also undesirably concurrently etch in a generally horizontal direction underneath the edges of the resist-coated areas; that is to say, the etchant solutions attack the side walls of the depressed etched areas. This is commonly referred to as undercutting.
Undercutting is undesirable, because it causes the cross-sections of the remaining unetched copper areas to be non-uniform and non-predictable; under some conditions removes edge support for the remaining unetched copper areas; reduces the effective current-carrying capacity of the remaining unetched copper areas; in some cases, completely removes the copper; and may cause shorting between adjacent remaining unetched copper areas when metallic etch resists are used, these tending to break away as slivers to connect adjacent circuits; etc.
Undercutting presents a particular problem when one is etching very dense, fine line circuits. The electronic industry is constantly striving to put more components on a printed circuit board and to make the components themselves more complicated. To connect all these components more and more circuits are packed into a given area. The only way to fit them in is to make the circuit lines narrower and closer together. As the circuit lines become narrower, a fixed amount of undercutting becomes more significant; i.e. becomes a larger percentage of circuit line width.
To achieve a reduction in the amount of undercutting, many parameters had to be met. These include:
1. Single Liquid Phase
The etchant should be a single phase liquid, preferably aqueous, system. Most undercut controlling formulations used in the printing industry rely on an oleophilic member coupled to an aqueous acid etchant. The nature of current spray etching equipment technology does not allow for practical application of such a two-phase etching system.
2. Active Components Must Not Attack Polyvinyl Chloride
Since most etching equipment in the printed circuit industry is manufactured from one or more varieties of polyvinyl chloride, an active undercut-inhibiting chemical component must not be a compound that would attack or soften polyvinyl chloride.
3. No Reduction in Etch Rate
To advance the printed circuit quality etching art at the cost of reduced production output would be self-defeating. Unfortunately, many undercut inhibiting components reduced etch rate.
4. No Increase in Cost Per Ounce of Copper Etched
Some undercut inhibitors may have advantages in undercut control but are so costly as not to be marketable.
5. Formulate Etchant to Operate at Higher Copper Concentrations
In order to further increase the cost efficiency of the new product it would be advisable to operate at 20 to 22 ounces per gallon of copper as compared with the prior 16 to 18 ounces/gallon ammoniacal alkaline cupric etchant formulations.
It is known to reduce undercutting by minimizing the time that the copper remains in the etchant solution, and also by operating the ammoniacal alkaline cupric etchant solution at a predetermined optimum chemical balance. However, despite these precautions, some undercutting still occurs, thereby resulting in poor electrical performance and poor mass production reliability.
U.S. Pat. No. 4,116,699 discloses an organic additive for use in acidic aluminum polishing solutions to prevent transfer etch after the substrate has been removed from the etchant solution. Although generally satisfactory for its intended purpose, the aluminum polishing solution disclosed in the aforesaid patent has not been demonstrated to be effective in ammoniacal alkaline cupric etchant solutions, nor has it been shown to prevent undercutting, particularly when the substrate is in the etchant bath. Further, the aluminum polishing solution disclosed in this patent simply reduces the attack of the etchant on the total surface and does not produce a selective reduction of etching on a vertical surface without reducing the etching on a horizontal surface.
The use of formamidine disulphide is described in U.S. Pat. No. 3,033,725 as a passivating agent to prevent undercutting in copper etching solutions. U.S. Pat. No. 3,287,191 describes the use of an additive that reduces undercutting in printed circuit etching, wherein a metal more noble than copper has been deposited upon the copper. U.S. Pat. No. 3,161,552 describes the use of an additive which controls the character of the protective film which is used to prevent undercutting. All three of the aforementioned patents disclose undercutting inhibitors which are used in acid etching baths. Acid copper chloride baths etchs copper by the following ionic reaction: ##STR1## and can be regenerated by chlorine gas as follows: EQU 2Cu.sup.+ +Cl.sub.2 .fwdarw.2Cu.sup.++ +2Cl.sup.-
With proper controls, the etching rate can be maintained at a steady level. Unfortunately, the acidic nature of the etchant causes it to attack solder electroplating as rapidly as the copper itself. The only resists that can be used with acid etching are organic materials.